Filament assembly for an incandescent lamp comprising screw-in end plugs and method of manufature thereof

Abstract

 A filament assembly for an incandescent lamp includes a helically coiled filament (13) and a helically coiled end plug (10) engaging an end of the filament (13). The end plug comprises a coil having a first section (11) with a diameter and pitch adapted to engage in the end of the filament coil, and a second section (12) having a pitch different from the pitch of the first section. The second section (12) of the end plug also engages the outside of a current lead wire (14), to mount the filament on the lead wire. A method of manufacture of such a filament assembly is also described.

Description

[0001] This invention relates to a screw-in end plug for use in the filament assembly of an incandescent lamp.

[0002] The use of screw-in end plugs is commonplace in tungsten halogen lamp manufacture. The plugs are short sections of wound tungsten which screw in to the ends of a filament, providing a link between the active filament and the current lead. They are used routinely in monoplane, biplane and other multi-limb studio halogen filaments such as those made by GE Lighting.

[0003] When inserting the plugs into the filament coil to form the filament assembly, the level up to which the plug is screwed is important, since this will determine the active (or lit) length of filament wire in the assembly. This needs to be as closely controlled as possible since it is one of the major input variables which determines the primary lamp performance parameters (power consumption, light output, colour temperature, life etc.). Typically, this dimension is given a tolerance of ± 0.3mm, yet during assembly is controlled visually by judgement with the naked eye, hence is prone to error and demands a very high skill level.

[0004] Another important variable is the length of the end plug which is exposed below the filament. This needs to be controlled to typically ± 0.5mm to ensure that the filament fits the filament support bridge consistently with the correct amount of tension. At present, the length of exposed plug is determined by trimming with wire cutters, again using the naked eye to judge the trimming point.

[0005] Finally, in many cases, it is necessary to use a thick wire as the plug material to fill the space between the filament and the current lead. This is often necessary where there is a mismatch between the current lead diameter and the filament inside diameter. Often, in these cases, it is also necessary to wind the screw-in plug with a very close pitch. This makes processing difficult for two reasons: Firstly, the plug is very difficult to trim accurately without burring, and secondly, it is mechanically very inflexible, which makes it difficult to bend around a pre-bent current lead.

[0006] This invention enables each of the above problems to be addressed.

[0007] According to the present invention, there is provided a filament assembly for an incandescent lamp, including a helically coiled filament and a helically coiled end plug engaging an end of the filament, the end plug comprising a coil having a first section with a diameter and pitch adapted to engage in the end of the filament coil, and a second section having a pitch different from the pitch of the first section.

[0008] The invention also provides a method of assembling a filament assembly in accordance with the preceding paragraph, comprising screwing the first section of the filament plug into the end of the filament until a resistance is encountered as a result of the difference in pitch between the first and second sections of the end plug.

[0009] The end plug comprises a helical coil of wire, which differs from any existing plugs in that it comprises sections having at least two different pitches.

[0010] Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig 1 is a side view of a screw-in end plug of an incandescent filament lamp in accordance with the invention;

Fig 2 is a side view of part of a filament assembly of an incandescent lamp, showing one end of a filament mounted on a current lead wire by means of an end plug; and

Figs 3 and 4 show lengths of coiled wire from which end plugs may be cut.

[0011] Fig 1 shows an example of a screw-in end plug 10 which consists of a helical coil of wire, typically of tungsten, with a first section 11 have a first, relatively closely wound, pitch, and a second section 12 having a second, less closely wound pitch. The first and second sections are conveniently of the same diameter, but may be of different diameters.

[0012] The screw-in end plug 10 is used, as shown in Fig 2, to mount a filament 13 on a current lead 14. The first section 11 of the end plug has a pitch which enables the plug to screw into the end section of a given helical filament, whereas the second section 12 has a pitch which prevents the plug from being screwed beyond a pre-determined length inside the end of the filament 13. Thus, when the operator is screwing the end plug into the filament, a resistance is encountered as a result of the difference in pitch between the first and second sections of the end plug. Additionally, the pitch of the second section of the plug is such that it can be mounted onto the current lead wire 14 which may be bent up to an angle of more than 60 degrees.

[0013] Usually, in a practical manufacturing situation, individual screw-in end plugs are cut from a pre-wound length of coil, typically 150mm long. Such a pre-wound length of coil would, according to the present invention, comprise alternate lengths of coil of different pitches. This would lead to an additional advantage, namely that the operator could easily tell, by the boundary between two sections of different pitch, where to trim the plug once it is inserted into the filament coil. Figure 3 illustrates a typical length of pre-wound coil 20, where there are alternate sections 21, 22 of coil with roughly equal lengths of sections of different pitch. Figure 4 illustrates an alternative design of coil 20 where the sections 21, 22 of coil with different pitches have markedly different lengths.

Claims

1. Filament assembly for an incandescent lamp, including a helically coiled filament and a helically coiled end plug engaging an end of the filament, the end plug comprising a coil having a first section with a diameter and pitch adapted to engage in the end of the filament coil, and a second section having a pitch different from the pitch of the first section.

2. The filament assembly of claim 1, wherein said second section of the filament plug has a greater pitch than the first section.

3. The filament assembly of claim 1 or claim 2 including a current lead wire engaging in the second section of the filament plug.

4. Method of assembling a filament assembly in accordance with any one of claims 1 to 3, comprising screwing the first section of the end plug into the end of the filament until a resistance is encountered as a result of the difference in pitch between the first and second sections of the end plug.

5. The method of claim 4 including cutting the end plug from a length of coiled wire having alternating lengths with said first and second pitches, the cut being made at a position where the pitch changes.

Drawing